Alastair Clarke explains:
The theory is an evolutionary and cognitive explanation of how and why any individual finds anything funny. Effectively, it explains that humour occurs when the brain recognizes a pattern that surprises it, and that recognition of this sort is rewarded with the experience of the humorous response, an element of which is broadcast as laughter.
The theory further identifies the importance of pattern recognition in human evolution:
An ability to recognize patterns instantly and unconsciously has proved a fundamental weapon in the cognitive arsenal of human beings. The humorous reward has encouraged the development of such faculties, leading to the unique perceptual and intellectual abilities of our species.
http://www.eurekalert.org/pub_releases/2008-06/ph-maf062708.php